Layering with ENV based folder unclear

The docs indicate the ability to layer `tfvars` using env based folders [here](https://terraspace.cloud/docs/layering/full-layering/#env-folders) and [here](https://terraspace.cloud/docs/layering/extra/#layering). The first section (full-layering) seems to indicate this is "simple layering" but mentions the `TS_ENV` var without much explanation. The second section (extra) indicates the use of `TS_ENV` to organize based on env folders but doesn't explain.

Through trial and error I've learned that I need set both `TS_ENV` and `TS_EXTRA` like so: `TS_ENV=$TS_ENV; TS_EXTRA=$TS_ENV/base`. I think the Docs could use some added context here. Ideally, this should be supported out-of-the-box IMHO.

Start with the linked env-folders section in full-layering and the layering section in extra. Clarify how TS_ENV and TS_EXTRA work together for environment-based folders, including the setup described in the issue, and document whether this is supported out of the box.

Written by the indexing model from the issue text.